Copyright Statement

Abstract

Amoebic gill disease (AGD) is caused by Neoparamoeba perurans and represents a significant threat to Atlantic salmon marine farming in several countries worldwide. Sequential natural reinfection with N. perurans after treatment occurs after the first AGD outbreak during the grow-out phase of Atlantic salmon culture. Little is known about the immune response of Atlantic salmon following reinfection with N. perurans. In a previous single exposure study, using gill biopsies with various severity of AGD-lesion, we reported that the immune signalling in AGD-affected gills was highly dependent on the ratio of normal to hyperplastic gill tissues. Here, following experimental reinfection we investigated the transcriptional immune response in the gills of AGD-affected Atlantic salmon. Furthermore, we reported the inflammatory and immune response following a single exposure to N. perurans during late infection and compared it to the reinfected fish. Fish groups were selected based on the gross gill scores carried out during the trial. Two gill biopsies were collected from each AGD-affected individual, one with no lesion and one partly including AGD-lesion. Furthermore, gill biopsies were collected from uninfected controls. Pro-inflammatory and immune-related genes were characterised at the mRNA level by using a quantitative RT-PCR. Targeted immune genes included IL-1β, TCR-α chain, CD8, CD4, MHC-IIα, MHC-I, IgM and IgT. Histopathology and image analysis were used to assess the severity and to verify the reliability of the gross gill score as AGD severity assessment method to select fish groups for gene expression studies. Overall the expression at the mRNA level of the immune and pro-inflammatory genes analysed showed little change in AGD-affected gills of experimentally reinfected fish and of fish exposed once to N. perurans.